The invention relates to an adjuvant for the filtration of liquids ensuring the elimination of contaminating micro-organisms, which is constituted by polylactam powder obtained by polymerisation of monocyclic lactam by anion catalysis in a liquid medium. It is intended, in particular, for integration in a process for preparing drinkable water.
Drinkable water is obtained from water drawn from natural reserves, by decantation, filtration with sand beds and then chemical treatment using chlorine or ozone.
Improvements to these treatments are still under study, in particular with a view to preventing the taste of the water from being denatured by the chemical treatments and, above all, to improve its safety from the bacteriological viewpoint.
Research is primarily devoted, on one hand, to ultrafiltration or nanofiltration with membranes and, on the other hand, to filtration with activated charcoal and with doped activated charcoal.
The best activated charcoal currently used is coconut based (retention capacity from 4.107 to 1.108 E. coli bacteria per cm3). It has proved possible to obtain a better result with silver (pure metal)-doped activated charcoal using a technology developed by Cartis(copyright) (retention capacity from 1.5.108 E. coli bacteria per cm3). For reference purposes, sand has a retention capacity representing 2.106 E. coli/cm3 bacteria.
The object of the invention is to provide an alternative to activated charcoal, having a higher bacterial retention capacity and permitting simpler, hence less expensive, recycling.
The Applicant has discovered surprising properties in a polylactam powder which, in a hydrated condition, has a high capacity for adsorption and retention of bacteria, yeasts and parasitic micro-organisms.
The Applicant has thus come to contemplate its use as a filtre or as an adjuvant for filtration for decontaminating liquids and, most particularly, for the production of drinkable water.
The properties of the polylactam powder according to the invention are linked with the process for preparing it. This is a variant of a process that is known from, and described in, U.S. Pat. No. 3,061,592 (1962).
This patent describes a process for producing polyamides in the form of spheroidal granules, known as anionic polymerisation (that is to say making use of the opening of the lactam cycle and the generation of a carbanion).
This process includes placing a monocyclic lactam in solution in an organic solvent in which said lactam is soluble, adding a polymerisation catalyst of the alkali metal type and a polymerisation accelerator chosen from the organic isocyanates, carbodiimides and cyanamides, and adding a dispersing agent constituted by polyamide powder and talc. The process then includes heating the mixture at a temperature higher than the melting point of the lactam but lower than that of the polyamide, that is to say in the order of 100 to 150xc2x0 C. Following polymerisation, the product is hot filtered to remove the non-polymerised lactams, and then washed and dried.
The process is carried out in a liquid medium, by contrast with prior processes in which polymerisation was conducted xe2x80x9cin situxe2x80x9d in moulds, which necessitated subsequent mechanical treatment to reduce the mass of polymer to powder.
The patent specifies that the higher the polymerisation temperature, the finer the granules that are obtained and that, on the other hand, a reduction in the speed at which the mixture is stirred causes the formation of granules of larger sizes.
The powder used in the present invention is prepared by polymerisation by anionic catalysis in a liquid medium, as described in U.S. Pat. No. 3,061,592, using:
as a cyclic lactam monomer, lactam 6 or caprolactam, lactam 12 or lauryllactam, or a mixture of the two;
and, as a dispersing agent, micronized silica, which plays the part of a polymerisation initiator. The polymer powder thus obtained has the following characteristics:
it is constituted by particles having, when observed using electron microscopy, a characteristic appearance of small clusters of agglutinated spheres (and not of separate spheres, as with other, similar polymers) with a size of 20 to 80 microns;
these particles present numerous micropores, with sizes ranging from 0.01 to 3 microns.
As will emerge from the examples, the powder according to the invention can be used in a column the lower part of which has a layer of sintered glass and of glass wool; before it is used, the powder must be hydrated to ensure that it is uniformly distributed in the column; the powder is introduced into the column as an aqueous suspension and, after decantation, it is maintained with a surplus of liquid above its upper surface.
The column thus prepared enables liquids artificially contaminated with bacteria or other micro-organisms to be filtered in order to measure the retention capacity of the powder.
The Applicant has thus been able to demonstrate that the powder can retain 107 E. coli bacteria per gram of powder, by filtration of a suspension inoculated with 106 bacteria/ml, with these extreme conditions corresponding to the saturation level of the powder.
Contrary to what is observed in the case of activated charcoal, bacterial retention capacity is not proportional to the specific surface area of the particles, which is only 15 m2/gram; it is far greater, thanks to the existence of micropores inside the particles.
The exceptional bacteria retention capacities of polylactam powder thus brought to light make it a material highly suitable for an additional or alternative step in conventional liquid filtration processes and, more particularly, for making water drinkable by ensuring the microbial decontamination of water taken from a given natural reserve.
The invention thus relates to the use of a polylactam powder as a filtration adjuvant, to replace, or serve as a complement to, the conventional filtration steps currently used in drinkable water preparation processes and, in particular, to replace chemical treatment and/or to replace the activated charcoal filtration step.
Polylactam powder is, furthermore, particularly advantageous as it can be recycled by simple sterilization.
The use of polylactam power can easily be applied on any scale: it can be integrated in an industrial filtration process using columns or cartridges including several layers of filtering material, a process ensuring the purification of water intended for common distribution; it can also be integrated in a portable device also comprising several layers of filtering materials, of the type having a column or a straw for sucking drinks, designed for the decontamination of individual drinking water, in particular for travellers not having access to water of bacteriologically safe quality.
Polylactam powder can also be used as a step preliminary to water filtration, when the water has a high level of contamination. It is used in this case in a batchwise process, thus involving contact between the peviosly hydrated powder, and the contaminated liquid. After contact, with gentle stirring and of a duration sufficient to ensure adsorption of the bacteria, the solution is filtered using conventional methods, for example using a filter of sand or sintered glass, in such a way as to retain the polyfactam polymer that has adsorbed the bacteria and to recover the decontaminated water.
The invention thus also relates to the processes for preparing drinkable water comprising the use of polylactam powder as a filtration adjuvant with a high capacity of retention of microbial contaminants, either as an additional or alternative filtration step, or as a step serving as a preliminary to filtration, said processes being applicable both on an industrial scale and on an individual scale in the form of a portable device.